System and method for the broadcast dissemination of time-ordered data

ABSTRACT

A stream of time-ordered data, such as a movie, is divided into multiple fragments of equal length, which are repetitively transmitted at different respective repetition rates. The fragments are reordered for transmission so that those which occur near the beginning of the original data stream are transmitted more frequently than those which occur later in the data stream. When a user enters a request to utilize the data, the individual fragments are stored upon receipt at the user&#39;s premises, and reassembled into a contiguous stream. The ordering of the fragments is such that the wait time required before utilization of the data can begin is limited to a predetermined maximum, and at least one copy of every fragment becomes available by the time it is needed.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is divisional of U.S. patent application Ser. No.13/235,740, entitled SYSTEM AND METHOD FOR THE BROADCAST DISSEMINATIONOF TIME-ORDERED DATA, filed Sep. 19, 2011, which is a divisional of U.S.patent application Ser. No. 12/388,557, entitled SYSTEM AND METHOD FORTHE BROADCAST DISSEMINATION OF TIME-ORDERED DATA, filed Feb. 19, 2009(now U.S. Pat. No. 8,046,818), which is a continuation of U.S. patentapplication Ser. No. 11/106,173, entitled SYSTEM AND METHOD FOR THEBROADCAST DISSEMINATION OF TIME-ORDERED DATA, filed Apr. 13, 2005 (nowU.S. Pat. No. 7,565,681), which is a continuation of U.S. patentapplication Ser. No. 09/414,514, entitled SYSTEM AND METHOD FOR THEBROADCAST DISSEMINATION OF TIME-ORDERED DATA, filed Oct. 8, 1999 (nowU.S. Pat. No. 7,155,735), each of which is incorporated herein byreference in its entirety for all purposes.

TECHNICAL FIELD

The present invention is directed to systems for broadcastingtime-ordered data, such as streaming media presentations, and moreparticularly to a system which minimizes the delay between a user'sindication of a desire to receive the data and the commencement of theutilization of the data, with efficient use of the broadcast bandwidth.

BACKGROUND

A variety of situations exist in which users desire access totime-ordered data, and prefer to begin utilizing that data as soon aspossible after they have entered a request for the data. One example isthe reproduction of streaming media data, such as a video presentationor a musical rendition. In the case of a video, for instance, atelevision system subscriber may desire to view a particular movie thatis being broadcast by a cable or satellite television network.

In a conventional television system, the subscriber is required to waituntil the scheduled time at which the movie is to be broadcast before itis possible to begin viewing it. For instance, a given movie may berepetitively broadcast on a particular channel every two hours. If thesubscriber becomes aware of the movie's availability one half hour afterthe most recent broadcast has begun, it is necessary to wait an hour anda half before the movie can be viewed in its entirety from start tofinish. As an alternative, the subscriber could choose to ignore thefirst half hour that was missed, and view the remaining portion of themovie. In many situations, however, this alternative is unacceptable,particularly where the initial portion of the movie is essential to anunderstanding of events that occur later in the movie.

To alleviate this situation, different approaches have been employed toreduce the time that a subscriber must wait until the next instance atwhich the beginning of the movie becomes available. In general, themaximum potential waiting period is reduced by allocating a greateramount of bandwidth, e.g., number of channels, to the movie. In certainsituations, the available bandwidth is sufficient to provide truevideo-on-demand to all viewers, whereby the movie begins to betransmitted in its entirety to any viewer immediately upon receiving arequest for the movie. In practice, however, this capability can only becost-effective in a relatively small, closed environment, such as withina hotel or an apartment building. In these situations, where the numberof viewers at any given time is limited, it is possible to effectivelydedicate a channel to each viewer and to begin a transmission of themovie over a viewer's channel upon receipt of a request to begin themovie.

This approach to video-on-demand services is not economically feasiblein a broadcast environment, where the number of potential viewers is solarge that the required bandwidth becomes cost-prohibitive.Consequently, broadcasters employ an approach that is labeled“near-video-on-demand”. In this approach, a popular movie is broadcastat multiple staggered start times over a limited number of channels. Forinstance, a two-hour movie can be broadcast over four channels at 30minute intervals. Consequently, the viewer never has to wait more than30 minutes for the start of the next presentation. With this approach,the number of channels occupied by a given movie is inverselyproportional to the wait period. If 5 minutes is considered to be anacceptable wait time, a two-hour movie would require 24 channels. Aone-minute wait time would occupy 120 channels.

In an effort to simulate true video-on-demand capabilities in anear-video-on-demand environment, a class of techniques has beendeveloped which rely upon preliminary storage of part of the movie atthe viewer's premises. One example of this technique is described inU.S. Pat. No. 5,884,141. In this technique, a desired movie istransmitted over multiple channels at staggered times, as innear-video-on demand. If the staggered presentation is to begin every 30minutes, for example, the first 30 minutes of the movie, known as theleader, is automatically stored at the subscriber's premises. When asubscriber enters a request to view the movie, for example by pressing a“play” button on a set-top converter or a remote control unit, or bydialing a specified phone number, the first 30 minutes of the movie isreplayed from the locally stored leader. During this time, the remainingportion of the movie, which occurs after the leader, begins to be storedfrom one of the channels allocated to the movie, and is replayed in atime-shifted fashion after the replay of the stored leader hascompleted.

Using this approach, the number of movies that can be viewed withsubstantially no wait time is determined by the amount of local storagecapacity at the subscriber's premises. For example, several gigabytes ofstorage in the set-top converter unit are required to cache a 30-minuteleader for each of ten select movies. If the subscriber desires to viewany movie other than the ten whose leaders have been stored, the normalnear-video-on-demand wait time would be encountered, e.g., 30 minutes.

While this approach can significantly reduce viewer wait times, relativeto conventional and near-video-on-demand transmissions, it is highlydependent upon the amount of storage capacity that is available at theuser's premises. It is desirable to provide video-on-demand capabilitiesin a broadcast environment with significantly reduced storagerequirements at the subscriber's premises, and without asubscriber-hardware-based limit on the number of broadcast presentationsthat can be viewed with minimal wait times.

A broadcast technique which offers such a possibility is described inU.S. Pat. Nos. 5,751,336 and 5,936,659, as well as related publications“Pyramid Broadcasting for Video On Demand Service,” by S. Viswanathanand T. lmielinski, SPIE Proceedings, Vol. 2417, pp. 66-78, February1995, and “Metropolitan Area Video-On-Demand Service Using PyramidBroadcasting” by Viswanathan and lmielinski, Multimedia Systems, Vol. 4,pp. 197-208, 1996. In the technique described in these documents, eachmovie is divided into segments of increasing size, and each segment isrepeatedly transmitted in an associated logical channel of the broadcastmedium. In the disclosed implementations, each segment is approximately2-2.5 times longer than the preceding segment. At the receiving end,once the first segment is received, all of the other segments arereceived in time, so that continuous viewing of the movie is possible.

While this technique provides improved results relative to theapproaches described previously, the particular embodiments disclosed inthe patents and related publications still require a significant amountof bandwidth. It is an objective of the present invention to improveupon the basic principles described in these references, and providemore efficient bandwidth utilization while minimizing viewer accesstime. It is a further objective to address some of the practicalproblems associated with the implementation of this technique which arenot addressed in the references.

SUMMARY

In accordance with the present invention, the foregoing objectives areachieved by dividing a stream of time-ordered data into multiplefragments of equal length, and repetitively transmitting the fragmentsat different respective repetition rates. The fragments are reorderedfor transmission so that those which occur near the beginning of theoriginal data stream are transmitted more frequently than those whichoccur later in the data stream. When a subscriber enters a request toutilize the data, e.g., view a movie, the individual fragments arestored upon receipt at the subscriber's premises, and reassembled into acontiguous stream for the subscriber. The ordering of the fragments issuch that the wait time required before utilization of the data canbegin is limited to a predetermined maximum, and at least one copy ofevery fragment becomes available by the time it is needed.

Various techniques are provided by the present invention that offerpractical advantages for the broadcaster and the subscriber within thisgeneral mode of operation. In one approach, a conventionally transmitteddata stream, in which each of the fragments appear in their sequentialorder, accompanies the reordered transmission. This sequentially orderedstream of data can be viewed in a conventional manner by subscribers whodo not have the local storage required to decode the reorderedtransmission. Some of the fragments that appear within theconventionally transmitted data stream are employed as substitutes, orproxies, for some of the reordered fragments. As a result, fragments canbe deleted from the reordered transmission, to make the overalltransmission periodic and thereby allow a given program to be broadcastindefinitely by repeatedly transmitting a finite data stream. In afurther implementation, this approach can be used to reduce the timethat users must wait between the end of the availability of one movieand the time at which the viewing of a new movie can begin.

Further features of the invention relate to encoding techniques thatprovide for effective use of the available bandwidth, and an approach tomultiplexing which accommodates variable data rates within a fixedbandwidth transmission. These and other features of the invention, aswell as the advantages provided thereby, are explained in greater detailhereinafter with reference to exemplary embodiments of the inventiondepicted in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of a broadcast system of the type inwhich the present invention can be implemented;

FIG. 2 is a time line depicting a sequence of fragments in apresentation;

FIG. 3 illustrates one example of the manner in which a presentation canbe divided into segments for allocation to transmission substreams;

FIG. 4 is a plot of the separation of fragment copies versus playbacktime for the embodiment of FIG. 3;

FIG. 5 is a plot of optimal and quantized fragment densities;

FIG. 6 is an enlarged view of a portion of the plot of FIG. 5,illustrating a differential change in the length of a presentationsegment;

FIG. 7 is a block diagram of a first embodiment of a decoder forprocessing data that is transmitted in accordance with the presentinvention;

FIG. 8 is an illustration of a format for transmitting one fragment ofdata;

FIG. 9 is a block diagram of a second embodiment of the decoder;

FIGS. 10 a-10 c illustrate the concepts which underlie the deletion offragments from substreams and the use of proxy fragments;

FIG. 11 illustrates the relationship of encoded substreams to aconventional stream;

FIG. 12 illustrates the alignment of deleted fragments relative to proxyfragments in a conventional stream;

FIG. 13 illustrates the switching of encoded substreams from onepresentation to another;

FIG. 14 is an alternative illustration of the switching of encodedsubstreams from one presentation to another;

FIG. 15 is an illustration of a first embodiment of switching whichemploys reordered fragments, without a conventional layer;

FIG. 16 illustrates a second embodiment of switching with reorderedfragments, which includes a non-contiguous conventional layer;

FIG. 17 illustrates another embodiment of switching with reorderedfragments, which employs a contiguous conventional layer;

FIG. 18 is a time diagram illustrating the relationship between thereceipt and play of fragments;

FIG. 19 is an illustration of switching with a conventional layer, usingirregular stop and start boundaries;

FIG. 20 is an illustration of the assignment of nominal times tofragments; and

FIG. 21 is a graph depicting the results obtained by different broadcasttechniques.

DETAILED DESCRIPTION

Generally speaking, the present invention comprises a set of techniquesfor the efficient operation of a system which provides virtual on-demandaccess to temporally-ordered data that is disseminated via a broadcastmedium. To facilitate an understanding of the principles which underliethe invention, it is described hereinafter with reference to a specificapplication thereof. In particular, reference is made to the use of theinvention in the context of streaming media presentations, such astelevised movies. It will be appreciated, however, that the practicalapplications of the invention are not limited to these particularexamples. Rather, the invention can be employed in any situation inwhich it is desirable to provide temporally-ordered data in a mannerwhich permits a user to enter a request to receive the data at anyarbitrary point in time, and to begin utilization of the data withminimal delay after such a request.

In the context of the present invention, the term “temporally-ordereddata” refers to any collection of data in which some portion of the datamust be received prior to the time that another portion of the data canbe utilized. For instance, in the case of a video presentation, theframes of a movie can be received in any order, and stored forsubsequent presentation. However, the viewer cannot begin to watch themovie until the first frame has been received. The invention guaranteesthat the first frame is available with a minimal time delay, and eachsucceeding frame is also available by the time it is needed to recreatethe original sequence. In another application, the invention can beemployed to broadcast software programs, and similar types of data, inwhich some portions of the program are required for the user to beginits operation, whereas other portions may not be required until later inthe operation, and can therefore be received after the operation hasbeen initiated. In yet another application, the invention can beemployed to broadcast media objects such as audio, video or animation inthe context of a multimedia presentation.

The basic objective of the present invention is to reduce the wait timethat is experienced by users between the entry of a request to receivedata, such as a movie, and the time when the utilization of the data inits time-ordered fashion can begin, while minimizing the amount ofbandwidth and local data storage that is needed to support such acapability. The techniques of the invention are implemented in a systemin which the temporally-ordered data is divided into a sequence of smallequally-sized fragments, and the fragments are repetitively transmittedin one or more streams over a suitable broadcast medium, such that thefragments at the beginning of the presentation are transmitted at arelatively high density, whereas those near the end of the presentationare transmitted with a lower density. To facilitate an understanding ofthe principles which underlie the invention, the basic concept of such asystem is first described with reference to relatively simple examples,followed by more detailed discussions of various optimizations that areprovided by the invention. In the following discussion, a stream of datawhich is transmitted in accordance with the foregoing principle, wherefragments are re-ordered with differing densities, is identified as an“encoded” stream, whereas a stream of data in which the fragments aretransmitted in their original sequential order is identified as a“conventional” stream.

General Concept

Referring to FIG. 1, an encoded stream of data fragments whichconstitute a video presentation is transmitted from a source 10, such asa cable head-end transmission station, to multiple subscribers'premises. At each subscriber's premises, the stream of fragments isreceived in a suitable set-top converter 12, or other equivalent type ofequipment for receiving signals from the source. A decoder 14 within theconverter reassembles the fragments into a continuous video stream, andstores them in a suitable frame buffer 16, where they are sequentiallypresented to the subscriber's television receiver 18. The ordering ofthe fragments in the encoded stream is such that, regardless of anyarbitrary point in time at which any subscriber enters a request to viewthe presentation, the first fragment of the presentation is availablewithin a maximum period of time τ, and at least one copy of any givenfragment becomes available by the time it is needed for viewing in theproper order.

FIG. 2 is a time line which illustrates the relationship of variousfactors that are employed throughout the following discussion. Time 0 isconsidered to be the moment at which the subscriber presses a “play”button or performs an analogous action to enter a request to view apresentation or otherwise utilize temporally-ordered data. Time instantt(0) is the moment at which the display of the first fragment in themedia presentation begins, τ seconds later. Each fragment n of thepresentation is displayed at a corresponding time t(n). The followingvariables are also employed in the discussion of the principles whichunderlie the invention:

-   -   G Size of a fragment    -   n_(max) Index of the last fragment in the presentation    -   T Total running time of the presentation    -   v(n) Fragment density, i.e., the number of times the n-th        fragment appears in a data stream of length T    -   η A bandwidth multiplier    -   m(n) Nominal bandwidth of the presentation at fragment n.

The bandwidth multiplier, or expansion coefficient, η describes theamount of bandwidth that is needed to provide a wait time τ, compared tothe amount of bandwidth B that would be required for a conventional datastream. For instance, a movie might be transmitted at a nominal bit rateof 3 Mb/s. Thus, a bandwidth multiplier of 9 indicates a bit rate of 27Mb/s. The bandwidth multiplier can be expressed as a function of T andτ. To simplify the initial discussion of the principles which underliethe invention, it is assumed that the repetitively transmitted copies ofa given fragment are uniformly distributed in time. Furthermore, it willbe assumed that G is a constant, i.e. all of the fragments are of equalsize. In the case of a media presentation, the size of a fragment can bethought of in terms of a time increment of the presentation, e.g. 0.5second. When considering transmission and storage issues, however, thesize of a fragment is preferably expressed as an amount of data. In oneembodiment, each fragment contains 188 bytes of data.

Each fragment n is transmitted repeatedly, ideally at time intervals oflength (T/ν(n)). Consequently, the first occurrence of a fragment n isno later than (T/ν(n)). To guarantee contiguous assembly of thefragments at the viewer's premises, the fragment must be received nolater than t(n). Therefore, the fragment density must conform to thefollowing relationship:(T/ν(n))≦t(n)  (1)orν(n)≧T/t(n)  (2)Thus, the fragment density, or repetition rate, is a decreasing functionof n, the fragment's location within the original presentation.

A lower bound on the bandwidth multiplier is given by the mean fragmentdensity:

$\begin{matrix}{\eta \geq {\frac{1}{T}{\int_{0}^{T}{\left( \frac{T}{t + \tau} \right){\mathbb{d}t}}}}} & (3) \\{= {\ln\left( {1 + \frac{T}{\tau}} \right)}} & (4) \\{\approx {\ln\left( \frac{T}{\tau} \right)}} & (5)\end{matrix}$Encoding

The fragment stream is encoded so that the individual fragments repeatwith a density that satisfies the relationship defined at (2) above. Avariety of different encodings are possible which comply with thisrelationship. In one embodiment, the presentation is divided into aplurality of segments, and each segment is repetitively transmitted as asubstream of fragments. Each of the individual substreams can betransmitted in parallel with all of the other substreams. Morepreferably, however, all of the substreams are transmitted in atime-division multiplexed manner, to form a complete data stream ofencoded data.

The first substream contains a repeating loop of the lowest-numberedfragments in a presentation, e.g., the earliest frames in a movie. Themaximum length of that segment is defined as the maximum length of timeby which successive copies of the same fragment can be separated andstill meet the constraints of relationship (2). As many fragments aspossible are contained in the segment, until that limit is reached. Thenext succeeding substream contains a repeating loop of thelowest-numbered fragments that are not in the preceding substream. Thissegment can be at least as long as the preceding segment, and typicallywill be longer because the maximum spacing between its lowest-numberedfragment will be greater. Successive segments are determined in thismanner, until the entire presentation is encoded. The number ofsubstreams, which corresponds to the number of segments, determines thebandwidth multiplier η.

This encoding scheme will first be explained with reference to anexample that is orderly, and which corresponds to the embodimentsdisclosed in the previously cited references. The first fragment in asegment i is labeled with the index n_(i), where n(0)=0. There are atotal of N segments in the presentation. In this first example, eachsubstream i is allocated the same amount of bandwidth as that whichwould normally be allocated to the presentation when it is transmittedin a conventional, i.e., time-ordered sequential, manner. Typically,this bandwidth is about 3 Mb/s for a video presentation that iscompressed according to the MPEG encoding standard.

FIG. 3 illustrates the manner in which the segments are derived from theoriginal presentation in this initial example, for allocation torespective substreams. The presentation is divided into a sequence ofn_(max) fragments. Since the maximum wait time is defined as τ, thebeginning of the presentation must be available at least every τseconds. Therefore, the initial segment contains the first r seconds ofthe presentation, in this case fragments n₀ through n₁−1. The firstsubstream, i.e., Substream 0, consists of a repeating copy of thissegment.

The beginning of the remainder of the movie must be available at leastevery 2τ seconds, i.e., the initial wait period τ plus the length of thepreceding segment, which is equal to τ. Accordingly, the second segmentcontains the next 2τ seconds of the movie, in this case fragments n₁through n₂−1. The second substream comprises a repeating copy of thissecond segment.

The beginning of the next segment, which is 3τ seconds into thepresentation, must be available every 4τ seconds, i.e., the length ofthe wait time τ, the first segment τ and the second segment 2τ.Therefore, the next segment contains the next 4τ seconds of thepresentation, which is repeatedly transmitted in the third substream.Continuing on in this manner, each successive segment is twice as longas the previous segment, and is therefore transmitted in its respectivesubstream at one-half the repetition rate of the previous segment.

After N segments have been obtained, the total length of thepresentation is (2^(N)−1)τ. For instance, by using seven substreams ispossible to transmit a movie of length 127τ. Referring to Equation 5,the optimum bandwidth multiplier η for a movie of length 127τ isIn(127), or 4.84. Hence, an encoding scheme of the type described above,which requires seven substreams, and hence where η=7, uses morebandwidth than that which is optimal.

FIG. 4 illustrates a plot of the maximum separation between copies of afragment versus the time at which that fragment is expected to bepresented. The solid line illustrates the encoding example describedabove, and the dashed line depicts the theoretical optimum defined byEquation 5. This plot reveals that excess bandwidth is consumed byportions of the presentation that are repeated more often thannecessary. For instance, a fragment of the presentation which occursjust prior to 3τ seconds into the presentation can, in theory, beavailable about every 4τ seconds. However, in the foregoing encodingscheme, it is repeated nearly every 2τ seconds. Excessive bandwidthrequirements occur when a segment is too long, since the start and endof the segment can have vastly different requirements with regard to themaximum separation of successive copies of a fragment that is needed tosatisfy relationship (2).

In the orderly embodiment described above, that separation varies by afactor of two within a given segment. More optimum encoding schemes canreduce that variation, through the use of shorter segments, which inturn reduces the bandwidth that is allocated to each substream. Forinstance, each substream could be allocated one-tenth of the bandwidththat is allocated to a conventional video signal stream, e.g., 0.3 Mb/s.In this case, therefore, the first substream requires only enoughbandwidth to repetitively transmit the first 0.1τ seconds of thepresentation. The beginning of the remainder of the presentation must beavailable every 1.1τ seconds, i.e., the length of the wait time τ plusthe length of the first segment. Therefore, the second segment comprisesthe next 0.11τ seconds of the presentation. The beginning of the thirdsegment must be available every 1.21τ seconds, so this segment comprisesthe next 0.121τ seconds of the movie.

In this approach, each successive segment is 10% longer than itspredecessor. After N segments have been obtained, the total length ofthe presentation, T, can be defined as:

$\begin{matrix}{T = {0.1\tau{\sum\limits_{i = 0}^{N - 1}(1.1)^{i}}}} & (6) \\{= {0.1{\tau\left( \frac{(1.1)^{N} - 1}{0.1} \right)}}} & (7) \\{= {\left( {(1.1)^{N} - 1} \right)\tau}} & (8)\end{matrix}$By means of this approach, a presentation of length 128τ can betransmitted in 51 substreams. Since each substream is 1/10 the bandwidthof a conventional video stream, the bandwidth multiplier η equals 5.1.It can be seen that this encoding scheme is substantially closer to theoptimal bandwidth of Equation 5 than the orderly example describedpreviously, where η=7.

The foregoing approach can be generalized by allocating each substreaman amount of bandwidth λB, where B is the bandwidth of a conventionalvideo stream, e.g., 3 Mb/s, and λ is a positive number in the range from0 to 1. In a preferred embodiment of the invention, λ is the same forevery substream, i.e. each substream receives equal bandwidth. In thiscase, the total length of the presentation is defined by the followingpower series:

$\begin{matrix}{T = {{\lambda\tau}{\sum\limits_{i = 0}^{N - 1}\left( {1 + \lambda} \right)^{i}}}} & (9) \\{= {{\lambda\tau}\left( \frac{\left( {1 + \lambda} \right)^{N} - 1}{\lambda} \right)}} & (10) \\{= {\left( {\left( {1 + \lambda} \right)^{N} - 1} \right)\tau}} & (11)\end{matrix}$and the bandwidth multiplier η is:

$\begin{matrix}{\eta = {\lambda\; N}} & (12) \\{= {\frac{\lambda}{\ln\left( {1 + \lambda} \right)}{\ln\left( {\frac{T}{\tau} + 1} \right)}}} & (13)\end{matrix}$

The factor ln(T/τ+1) is equal to the optimal bandwidth multiplier setforth in Equation 4. The excessive bandwidth is therefore represented bythe factor λ/ln(1+λ), where a value of unity corresponds to optimal.Hence the excess bandwidth requirements shrink as λ is reduced towardzero, i.e. decreasing λ reduces the required bandwidth. Preferably, λ isless than or equal to ½, and in practical embodiments of the invention⅓≦λ≦ 1/25.

The foregoing analysis assumes that each segment i has an optimal lengthcorresponding to a repetition period of:p(i)=τ(1+λ)^(i).In practice, however, it may not be possible to achieve such an optimumvalue for each segment. In particular, if the substreams are to betransmitted using a simple form of time-division multiplexing, eachsegment must contain an integral number of equal-length fragments. Thisrequirement imposes a quantization constraint on the lengths of thesegments. In effect, this means that a quantity δ must be subtractedfrom the optimal length of the segment. The value of δ can be up to thelength of one fragment.

The foregoing analysis will now be reviewed, taking this quantizationconstraint into account. Since the first substream is allocated enoughbandwidth to repetitively transmit the first λτ seconds of thepresentation, the first segment has a length of λτ−δ seconds. Thebeginning of the remainder of the presentation must be available everyτ+(λτ−δ) seconds, which turns out to be (1+λ)τ−δ seconds. Therefore, thesecond substream can repetitively transmit the next λ(1+λ)τ−λδ secondsof the movie. Upon quantization, therefore, the second segment has alength of λ(1+λ)τ−(1+λ)δ seconds. Continuing on in this fashion, it willbe seen that the i-th substream repetitively transmits (λτ−δ)(1+λ)^(i)seconds of the presentation.

When the quantization factor is considered, the total length of thepresentation becomes

$\begin{matrix}{T = {\left( {{\lambda\tau} - \delta} \right){\sum\limits_{i = 0}^{N - 1}\left( {1 + \lambda} \right)^{i}}}} & (14) \\{\mspace{14mu}{= {\tau^{\prime}\left\lbrack {\left( {1 + \lambda} \right)^{N} - 1} \right\rbrack}}} & (15) \\{= {\left( {{\lambda\tau} - \delta} \right)\left\lbrack \frac{\left( {1 + \lambda} \right)^{N} - 1}{\lambda} \right\rbrack}} & (16)\end{matrix}$where

$\begin{matrix}{\tau^{\prime} = {\tau - \frac{\delta}{\lambda}}} & (17)\end{matrix}$

The following Table 1 illustrates a sample encoding for a 2-hourpresentation, such as a movie, in accordance with the foregoingconcepts. In this example, each fragment consists of 0.5 second, e.g.1.5 Mb, and the parameter λ equals ⅓, so that each substream isallocated approximately 1 Mb/s. The value of τ is 27 fragments, or 13.5seconds, and the quantization factor δ is 1.5 seconds, to provide amaximum wait time of 15 seconds. Since there are 22 substreams, theaggregate bandwidth requirement is 22 Mb/s, so that the bandwidthmultiplier η=7.33.

Each row of Table 1 corresponds to one substream. Hence, Substream 0consists of a repeating sequence of fragments 0 through 8, Substream 1consists of a repeating sequence of fragments 9 through 20, etc. Eachcolumn of Table 1 represents one time slot for the time-divisionmultiplexed transmission of all of the substreams. Thus, in the firsttime slot, fragments 0, 9, 21, 37, 58 . . . are transmitted, and in thenext time slot fragments 1, 10, 22, 38, 59 . . . are transmitted. Inpractice, the rows of the table continue indefinitely to the right,until such time as the transmission of the presentation is terminated.The encoded presentation is stored at the source 10, and repeatedlytransmitted over the substreams, as shown in Table 1.

Sub- stream Fragment Sequence 0 0 1 2 3 4 5 6 7 8 0 1 1 9 10 11 12 13 1415 16 17 18 19 2 21 22 23 24 25 26 27 28 29 30 31 3 37 38 39 40 41 42 4344 45 46 47 4 58 59 60 61 62 63 64 65 66 67 68 5 86 87 88 89 90 91 92 9394 95 96 6 123 124 125 126 127 128 129 130 131 132 133 7 173 174 175 176177 178 179 180 181 182 183 8 239 240 241 242 243 244 245 246 247 248249 9 327 328 329 330 331 332 333 334 335 336 337 10 445 446 447 448 449450 451 452 453 454 455 11 602 603 604 605 606 607 608 609 610 611 61212 811 812 813 814 815 816 817 818 819 820 821 13 1090 1091 1092 10921094 1095 1096 1097 1098 1099 1100 14 1462 1463 1464 1465 1466 1467 14681469 1470 1471 1472 15 1958 1959 1960 1961 1962 1963 1964 1965 1966 19671968 16 2619 2620 2621 2622 2623 2624 2625 2626 2627 2628 2629 17 35013502 3503 3504 3505 3506 3507 3508 3509 3510 3511 18 4677 4678 4679 46804681 4682 4683 4684 4685 4686 4687 19 6245 6246 6247 6248 6249 6250 62516252 6253 6254 6255 20 8335 8336 8337 8338 8339 8340 8341 8342 8343 83448345 21 11,122 11,123 11,124 11,125 11,126 11,127 11,128 11,129 11,13011,131 11,132 Sub- stream Fragment Sequence 0 2 3 4 5 6 7 8 0 1 • • • 120 9 10 11 12 13 14 15 16 • • • 2 32 33 34 35 36 21 22 23 24 • • • 3 4849 50 51 52 53 54 55 56 • • • 4 69 70 71 72 73 74 75 76 77 • • • 5 97 9899 100 101 102 103 104 105 • • • 6 134 135 136 137 138 139 140 141 142 •• • 7 184 185 186 187 188 189 190 191 192 • • • 8 250 251 252 253 254255 256 257 258 • • • 9 338 339 340 341 342 343 344 345 346 • • • 10 456457 458 459 460 461 462 463 464 • • • 11 613 614 615 616 617 618 619 620621 • • • 12 822 823 824 825 826 827 828 829 830 • • • 13 1101 1102 11031104 1105 1106 1107 1108 1109 • • • 14 1473 1474 1475 1476 1477 14781479 1480 1481 • • • 15 1969 1970 1971 1972 1973 1974 1975 1976 1977 • •• 16 2630 2631 2632 2633 2634 2635 2636 2637 2638 • • • 17 3512 35133514 3515 3516 3517 3518 3519 3520 • • • 18 4688 4689 4690 4691 46924693 4694 4695 4696 • • • 19 6256 6257 6258 6259 6260 6261 6262 62636264 • • • 20 8346 8347 8348 8349 8350 8351 8352 8353 8354 • • • 2111,133 11,134 11,135 11,136 11,137 11,138 11,139 11,140 11,141 •••

As noted previously, the quantization of the segments to contain anintegral number of fragments results in excess bandwidth requirementsbeyond the theoretical optimum set forth in Equation 4. FIG. 5illustrates the fragment density ν(n) over the length of the datastream. The solid curve depicts the theoretical optimum, and the steppedlevels indicate the quantized density function. The portion of each stepwhich lies above the theoretical optimum represents the excess bandwidththat is required by the quantization.

If the number of substreams N is known, it is possible to determine thelength of each segment which provides the most efficient use of theavailable bandwidth. This determination can be carried out recursivelyby means of a differential analysis, in which the best choice for t_(i),the starting point for segment i, is determined if t_(i−1) and t_(i+1)are known. Referring to FIG. 6, the bandwidth is at a local extremumwhen the two shaded areas in the diagram are of equal area, i.e.,[ν(t _(i−1))−ν(t _(i))]dt _(i)=(t _(i+1) −t _(i))[−ν′(t _(i))dt_(i)].  (18)Rearranging the terms indicates that the derivative of the grain densityat t_(i) must be equal to the slope of the dashed line in FIG. 6:

$\begin{matrix}{{v^{\prime}\left( t_{i} \right)} = \frac{{v\left( t_{i} \right)} - {v\left( t_{i - i} \right)}}{t_{i + 1} - t_{i}}} & (19)\end{matrix}$From this relation, t_(i+1) can be obtained as a recursion on t_(i) andt_(i−1):

$\begin{matrix}{{t_{i + 1} - t_{i}} = \frac{{v\left( t_{i} \right)} - {v\left( t_{i - 1} \right)}}{v^{\prime}\left( t_{i} \right)}} & (20) \\{\mspace{79mu}{= {{- \left( t_{i} \right)^{2}}\left( {\frac{1}{t_{i}} - \frac{1}{t_{i - 1}}} \right)}}} & (21) \\{\mspace{79mu}{= {{- \left( t_{i} \right)}\left( \frac{t_{i - 1} - t_{i}}{t_{i - 1}} \right)}}} & (22) \\{{{t_{i + 1}t_{i - 1}} - {t_{i}t_{i - 1}}} = {t_{i}^{2} - {t_{i}t_{i - 1}}}} & (23) \\{\frac{t_{i + 1}}{t_{i}} = \frac{t_{i}}{t_{i - 1}}} & (24)\end{matrix}$The bandwidth used by a substream is (t_(i+1)−t_(i))ν(t_(i))B, whichsimplifies to [t_(i+1)/t_(i))−1]B, a constant according to therecursion.

This analysis is based on the assumption that the data rate for apresentation is uniform. In practice, however, compressed data rates canvary in dependence upon the content of the presentation, whereinrelatively still scenes may require significantly less than 3 Mb/s,whereas action-packed scenes might require more than 6 Mb/s. Therefore,it is more useful to express the foregoing relationship in terms of thefragments of a segment rather than the starting time of the segment.

The first fragment of a segment i is represented as n_(i), and thesegment i contains all of the fragments in the range n_(i) to n_(i+1)−1.The fragment size G is the amount of memory required to represent onefragment of the presentation. A local extremum of the required bandwidthfor a segment i can be expressed as the following induction rule:

$\begin{matrix}{0 = {G{\frac{\mathbb{d}}{\mathbb{d}n_{i}}\left\lbrack {{\left( {n_{i} - n_{i - 1}} \right){v\left( n_{i - 1} \right)}} + {\left( {n_{i + 1} - n_{i}} \right){v\left( n_{i} \right)}}} \right\rbrack}}} & (25) \\{0 = {{v\left( n_{i - 1} \right)} - {v\left( n_{i} \right)} + {\left( {n_{i + 1} - n_{i}} \right){v^{\prime}\left( n_{i} \right)}}}} & (26) \\{{v^{\prime}\left( n_{i} \right)} = \frac{{v\left( n_{i} \right)} - {v\left( n_{i - 1} \right)}}{n_{i + 1} - n_{i}}} & (27)\end{matrix}$The range n_(i) to n_(i+1)−1 specifies which fragments should betransmitted in substream i. The time t(n_(i)), the time after pressing“play” at which the lowest-numbered fragment is expected to beavailable, specifies the maximum length of time to transmit one fullcopy of segment i in its corresponding substream. Although the inductionis derived with the values for n_(i−1) and n_(i+1), fixed and with n_(i)as a variable, the induction can be used to compute n_(i+1) from n_(i−1)and n_(i). This provides a way to compute n_(i) inductively in ascendingorder, with only one free variable. In particular, since choosing τfixes the curve ν(n), and by definition n₀=0, the number of fragments inthe zeroth substream, n₁, can be regarded as the free variable. Givenits choice, n₂ can be computed from n₀ and n₁, and so on.

One approach to choosing an appropriate encoding would be to iterateover all possible values of n₁, applying the induction recursive foreach, and choosing the one that wastes the least bandwidth. However,this solution may not be preferable because the high-numbered values ofn_(i) are extremely sensitive to the choice of n₁. Iterating over valuesof n₁ could miss near-optimal solutions, especially since in the vastmajority of candidate solutions generated in that manner, n_(N) is notlikely to match the length of the presentation.

A relaxation procedure is more preferably used to approximate thesolution given above. First, locally optimal values are computed for theodd-numbered n_(i)'s, given the even-numbered values. Then locallyoptimal values for the even-numbered n_(i)'s are computed, given theodd-numbered values. This procedure is iteratively repeated until thebandwidth stops improving.

Playback

At the subscriber's premises, the multiplexed substreams are receivedand the fragments are reassembled in sequential order, to display thepresentation. One embodiment of a decoder 14 for reassembling theencoded data is illustrated in FIG. 7. The received data is firstpresented to a gate 20, which determines whether a copy of each fragmenthas been received since the user entered a request to view thepresentation. If a given fragment has not been received since thesubscriber's request, it is forwarded to a buffer 22. If, however, acopy of the fragment has been previously received and stored in thebuffer, all subsequent copies of that fragment are discarded.

In order for the incoming signal stream to be correctly decoded, thegate must be able to identify each fragment that is being received,regardless of when the subscriber enters the request to view thepresentation. In one embodiment of the invention, an identifier isincluded with each transmitted copy of a fragment. FIG. 8 illustratesone possible format of a data packet for the transmission of fragments.To minimize bandwidth requirements, the presentation data is compressed,for example in accordance with the MPEG standard. The data packet for afragment therefore begins with a header 24 which conforms with the MPEGstandard. Following the header, a fragment identifier 26 is transmitted.In a straightforward implementation, the identifier could be the integerindex n which indicates the location of the fragment in the total datasequence. In other words, the first fragment in the presentation has anidentifier of 0, the next fragment is identified as 1, etc.Alternatively, the identifier could be a code value which the gate 20uses to look up the index number, by means of a previously downloadedlook-up table. This latter implementation might be preferable as amechanism for restricting access to authorized subscribers.

In addition to the index number or code value for the specific fragmentbeing transmitted, the identification data could also include a programidentification, or PID, which indicates the presentation to which thatfragment belongs. In many cases, the PID for a presentation isincorporated into the MPEG header information. In the event that it isnot, however, it can be included with the identifier portion 26 of thetransmitted data. Following the identifier 26, the actual data 28 forthe fragment is transmitted. In one embodiment of the invention, thetotal length of the data packet, including the header and identificationdata, is 188 bytes.

At the receiver, the gate 20 first examines the PID to determine whethera received fragment belongs to the presentation that has been requested.If the PID is correct, the gate examines the fragment identifier anddetermines whether that fragment has been previously stored in thebuffer 22. For example, the gate might have an associated bitmap 30, inwhich sequential bit positions respectively correspond to the fragmentsof the presentation. When a subscriber enters a request to view apresentation, the bitmap is reset so that all bits contain the samevalue, e.g., zero. As each fragment is received at the gate 20, thebitmap is checked to determine whether the bit corresponding to thatfragment's index number has been set. If not, i.e., the value is stillzero, the fragment data is forwarded to the buffer 22, for storage, andthe bit is set in the bitmap 30. Once the bit has been set, allsubsequent copies of that fragment are discarded by the gate 20.

Thereafter, at a time no later than τ seconds after the user has entereda request to view the presentation, a read circuit 23 retrieves thefragments stored in the buffer 22 in sequential order, and successivelyfeeds them to the frame buffer 16 at the proper video rates. Theencoding of the substreams ensures that at least one copy of eachfragment will have been received by the time that it is required forpresentation to the frame buffer 16. Thus, with a maximum wait time of τseconds, the subscriber is able to view the complete presentation, fromstart to finish.

In practice, the first copy of each fragment that is received after thesubscriber enters a request to view the presentation is stored in thebuffer 22. Hence, for at least a short period immediately after therequest is entered, every received fragment in each substream is goingto be stored. Thereafter, as copies of the fragments begin to berepeated, the received fragments are selectively discarded. Forinstance, in the exemplary encoding depicted in preceding Table 1, everyincoming fragment is stored for the first nine time slots. Thereafter,as segments begin to repeat, the rate of storage into the buffer 22begins to decrease.

In the embodiment of FIG. 7, the buffer 22 must be capable of writingthe data at the rate provided by the gate 20. Hence, it must be capableof operating at the maximum bit rate for transmission. It may not bedesirable, or economically feasible, to utilize memory which is capableof operating at such a rate, and which also has sufficient capacity tostore all of the fragments that will be needed. An alternativeembodiment of the decoder, which permits a slower form of storage to beemployed, such as a magnetic hard disk, is illustrated in FIG. 9. Inthis embodiment, a gate 34 receives each incoming fragment andselectively determines whether it is to be kept or discarded. Fragmentswhich are to be kept are forwarded to a fast FIFO buffer 36, which iscapable of writing data at the rate provided by the gate. This data isthen forwarded to a main buffer 38 at a slower rate, for storage untilneeded. In this embodiment, the main buffer 38 is sufficiently large tostore all of the necessary fragments, up to the entire length of thepresentation, and the fast FIFO buffer 36 can be much smaller. In analternate embodiment, fragments are discarded after being viewed. Inthis case, the main buffer 38 only needs to be large enough to store alimited portion of the presentation at one time, e.g. 44% of thefragments.

As a further feature of this embodiment, the gate 34 can make adetermination whether fragments that are to be kept will spend anegligible length of time in the buffers. For instance, immediatelyafter the subscriber enters a request to view the presentation, thelowest numbered fragments will be needed right away, to begin theplayback of the presentation. Later in the presentation, other fragmentsmay arrive just in time for display as well. In this case, therefore,the gate can forward these fragments directly to the read/assemblycircuit 40, so that they are immediately available for display. To doso, the gate might be provided with a running clock (not shown) thatindicates the number of the current fragment that is being displayed. Ifan incoming fragment is within a certain range of that number, it isdirectly forwarded to the read/assembly circuit 40.

In this mode of operation, the read circuit may receive fragments out oftheir sequential order. To accommodate this situation, a pre-buffer 42is located between the read/assembly circuit and the frame buffer 16.This pre-buffer provides random access writing capability, andsequential readout to the frame buffer 16.

To utilize the broadcast-video-on-demand capability provided by thepresent invention, it is necessary, therefore, that there be sufficientlocal storage capacity at the subscriber's premises to store andreassemble the fragments from the encoded transmissions. In some cases,however, viewers who do not have such storage capacity may also desireto see the presentation in a non-demand, or conventional, manner. Toaccommodate this situation, as well as provide additional advantages,discussed below, the bandwidth allocated to the presentation isincreased by the amount necessary to transmit the presentation in aconventional manner, e.g., in a sequential form at the nominal rate of 3Mb/s. In the conventional transmission, therefore, the fragments are nottransmitted at different rates, so that every fragment has the samerepetition period, which is approximately equal to T.

For the exemplary encoding depicted in the foregoing table, in which thebandwidth multiplier η=7.3, the addition of the conventional broadcastincreases the multiplier to a value of η=8.3. In other words, a totalbandwidth of 25 Mb/s would be allocated to the presentation. It is to benoted that, using known QAM-64 techniques, it is possible to transmitdata at a rate of approximately 27 Mb/s within a 6 MHZ channel band.Thus, both a conventional transmission of a 2-hour movie and an encodedtransmission with a maximum wait time of 15 seconds can be broadcastwithin the bandwidth of a single television channel. Throughout thefollowing discussion, embodiments of the invention in which aconventional, sequentially ordered copy of a presentation is transmittedwith the encoded substreams are identified as “layered” transmissions,i.e. one layer of substreams consists of the encoded data, and anotherlayer comprises the sequential data.

Periodic Transmission

In the transmission of the reordered data fragments, it is preferablethat the encoding of the presentation be periodic. In other words, thedata in all of the substreams should repeat at regular intervals. If theencoding meets this condition, the presentation source 10 only needs tostore one period of the encoding. Otherwise, it will be necessary toeither store, or generate in real time, an encoding that is long enoughto provide continuous service.

Furthermore, it is preferable to have the period of the encoding berelated to the length T of the presentation. In such a case, where aconventional layer of the presentation accompanies the encoded layer, asdescribed previously, they can be transmitted together with the sameperiodicity. This is accomplished by modifying each encoded substream tohave a period which is approximately equal to T. To illustrate themanner in which this can be accomplished, a section of one substreamwhich contains enough copies of a segment to span the period T isconsidered. If the length of that section exceeds the period T, aportion of one segment is deleted from the substream. If theconventionally transmitted version of the presentation does notaccompany the encoded version, the deletion of a portion of one segmentwould violate the condition set forth in relationship (2), since thedensity of the fragments in the deleted portion would be insufficient.For instance, FIG. 10 a illustrates a substream that complies with theconditions of relationship (2). In this substream, a given fragment,indicated by the shaded area, periodically appears in the substream. Themaximum separation between successive copies of the fragment complieswith the density requirements of relationship (2). FIG. 10 b illustratesa modified substream, in which a portion of a segment, which includesthe given fragment, has been deleted. As can be seen, the distancebetween successive copies of the given fragment exceeds the maximumseparation permitted by relationship (2).

However, it is possible to delete a portion of a segment if, for eachdeleted fragment, a copy of that same fragment is present at anappropriate location in another substream. In the context of thisdisclosure, a copy of a fragment that is present in another substream iscalled a “proxy”. In a practical implementation of the invention, theconventionally transmitted presentation can be used to provide proxiesfor deleted portions of encoded substreams. Referring to FIG. 10 c, whena proxy is present, it is possible to delete fragments if, for eachdeleted fragment, a proxy begins no earlier than one maximum separationperiod before the end of the next fragment copy in the substream, andends no later than one maximum separation after the end of the previouscopy of the fragment. This requirement ensures that the fragment, or itsproxy, fulfills the density conditions of relationship (2). The possiblelocations of a proxy, for the deleted fragment of FIG. 10 b, areillustrated in FIG. 10 c. As long as a copy of the deleted fragment ispresent in the proxy stream at one of these locations, it can be deletedfrom the original substream, since one copy will still be received atthe subscriber's premises by the required time.

When multiple contiguous fragments are deleted within a segment, thiscondition can be characterized more generally. If the number offragments to be deleted from a substream i is denoted as d_(i), then acopy of fragment j can be deleted from substream i only if a proxyexists in another substream with an offset Δ that lies in the range0≦Δ≦d_(i). A proxy offset is defined as the column number, ortransmission time slot, of the fragment copy to be deleted, minus thecolumn number of the copy which serves as a proxy.

Through the use of proxies in this manner, each substream can bemodified to provide it with periodicity T. FIG. 11 illustrates anexample of encoded substreams that are transmitted together with aconventional stream. To modify the encoded substreams to provideperiodicity T requires that up to one full segment, minus one fragment,may need to be deleted from each encoded substream. If the fragments aredeleted after the end of the period T, as indicated by the shading, therequirements of relationship (2) may be violated. However, the encodedsubstreams can be time shifted to permit deletion of a portion of asegment, and utilize proxies from the conventional stream. Referring toFIG. 12, the fragments are arranged in ascending sequential order withineach segment of an encoded substream. In the substream, a target rangeof five fragments, identified as A-E, is to be deleted, so that d_(i)=5.The substream is time shifted so that a copy of the first fragment inthe target range, i.e., fragment A, is aligned with a copy of anyfragment in the conventional stream that is within the target range. Inthis particular example, the conventional stream is time-divisionmultiplexed over three substreams.

Regardless of how the encoded substream is time shifted relative to theconventional stream, the first fragment always has the smallest proxyoffset, and the last fragment always has the largest proxy offset. Sincethe first fragment's proxy offset cannot be negative, and the lastfragment's proxy offset cannot exceed d_(i)−1, every proxy offset iswithin the required range. In the illustrated example, fragment A has anoffset of one, and fragment E has an offset of 3, which meets therequirements for removal of fragments. Thus, through appropriatedeletion of portions of segments, and time shifting of the conventionalstream and the substreams relative to one another to properly align theproxies of the deleted fragments, it is possible to ensure that eachencoded substream has a periodicity T.

In a case of a layered transmission, the periodicity of each substreamcan be made approximately equal to the total running time T of thepresentation. If layering is not employed, the periodicity of theencoded substreams can be made smaller than T. In this case, however,one or more additional substreams, which function as proxy substreams,need to be transmitted. Each encoded substream can be modified to have adesired periodicity, R, if a number of fragments equal to γ(t(n) mod R)are deleted from the substream. The length of the repeating portion ofthe substream is equal to t(n), and γ is equal to the number offragments that can be transmitted per unit time at bandwidth λη. For agiven R, therefore, the proxy substream must contain P fragments, whereP=γΣ_(n)(t(n) mod R). Since γ(t(n) mod R) falls within the range (0,γt(n)), the value for P therefore lies in the range (0, γT−N). The proxyfragments can therefore be transmitted in a number of proxy substreamsequal to the smallest integer at least as large as P/R. In most cases,the proxy substreams will not be full, which permits the requiredbandwidth to decrease when no proxy fragments are needed. It is possibleto employ proxies for up to half of the encoded fragments. As a result,encodings with periodicities which are considerably less than 0.5 T canbe accomplished.

Switching of Presentations

As long as an encoded presentation is continuously transmitted in such aperiodic fashion, a subscriber can view or otherwise utilize apresentation at any arbitrary time, with a maximum wait time of τ. In apractical implementation, however, the providers of broadcast serviceswill need to terminate the transmission of an encoded presentation, tochange to a new presentation. For instance, in a subscription televisionsystem, movies may be changed on a daily or weekly basis. Consequently,at a given point the encoded transmission of one movie will terminate,and the transmissions of the next movie will commence. This situation isschematically illustrated in FIG. 13, for an example where eachpresentation is transmitted over four substreams. Vertical lines withina substream indicate the boundaries between repeated segments. Forillustrative purposes, the segments are illustrated so that theirboundaries align at the time S when the switch is made from onepresentation to the other.

When this switch occurs, there exists a period of time during which asubscriber cannot enter a request to view the first movie and be able toreceive the entirety of that movie. This period begins when the lastcopy of any fragment in the first presentation is received at thesubscriber's premises. If the subscriber presses the “play” button anytime after the point at which it is possible to capture that copy of thefragment in its entirety, it is not possible to view the completepresentation. As a practical matter, this period of time has a durationwhich is equal to the length of the longest segment. In a further aspectof the invention, this period of time is minimized, to thereby reducethe length of time that a subscriber is prohibited from the viewing thefirst movie in its entirety and must wait for the start of the secondmovie.

The last point in time at which the user can press the “play” button andstill view the presentation in its entirety, before the switch to a newpresentation, is designated as the point L. Beginning at this point, itis only necessary to receive one copy of each fragment, to ensure thatthe entire presentation can be viewed in response to any actuation ofthe “play” button up to the point L. Any additional copies of fragmentsthat are transmitted after this time will not be used, and are thereforeunnecessary. FIG. 14 illustrates the transmission of one copy of eachfragment, i.e. one complete segment, in each substream, beginning attime L. As indicated by the blank areas, there is unused capacity in allbut the highest-numbered substream. If each segment continues to berepetitively transmitted, this unused capacity is occupied by redundantcopies of presentation segments. One way to reduce the duration of theperiod between L and S, therefore, is to move some of the fragments fromthe higher-numbered substreams into the available bandwidth oflower-numbered substreams, so that exactly one copy of each fragment istransmitted after the point L, using the full available bandwidth, asdepicted in FIG. 15.

In so doing, however, attention must be paid to the order in which thefragments are transmitted, to ensure that the fragment densityrequirements of relationship (2) are maintained. A straightforwardapproach that can be employed to ensure this condition is totime-division multiplex the substreams in their usual order, but to omitredundant copies of fragments. This approach guarantees that everyfragment will be at least as close to the last start time L as it wasbefore the redundant fragments were deleted. The transmission of thelast fragment n_(max) marks the point S at which the transmission of theencoded substreams for the new presentation can begin. In an un-layeredembodiment of the invention, the reordering of the fragments in thismanner provides a time period S-L that has a duration of T/λN. In thecase of the sample encoding depicted in Table 1 for a two hour movie,this results in a period of about 16.5 minutes during which thesubscriber cannot begin the playback of either movie.

In the case of a layered embodiment, the additional bandwidth providedby the conventional layer permits this period of time to be reduced evenmore. Two alternative approaches are possible, as respectivelyillustrated in FIGS. 16 and 17. In these figures, the conventionaltransmission is depicted by the horizontal layers on top of the encodedsubstreams. FIG. 16 illustrates the situation in which the finaltransmission of each fragment, represented by the shaded area, occursafter the conventional transmission has terminated. In this case, thebandwidth normally allocated to the conventional layer can also be usedfor the encoded transmission. In other words, the encoded transmissionhas a total available bandwidth of (λN+1)η. Consequently, the period S-Lis reduced to a duration of T/(λN+1). In the case of the sampleencoding, therefore, this results in a non-viewing period of about 14.5minutes for a two hour movie.

Of course, the implementation of FIG. 16 means that the conventionalviewing of the movie also has the same blackout period. In thealternative implementation of FIG. 17, the bandwidth allocated to thelast copy of each fragment is reduced, so that the conventionalpresentations can abut one another. This approach would appear to havethe same effect as the non-layered approach of FIG. 15, and lengthen theperiod S-L to T/λN for the encoded version of the presentation. However,using the conventional stream as a proxy, a sufficient number offragments can be removed from the encoded portion of the transmission toprovide a non-viewing period of duration T/(λN+1). In essence, all ofthe fragments in the last T/(λN+1) seconds of the presentation areavailable from the conventional layer, and are received at least asearly as they would be in the highest-numbered substream. Consequently,they can be omitted from the encoded substreams, thereby shortening theperiod of time necessary to transmit one copy of every fragment.

In each of the foregoing embodiments, the boundary between the firstpresentation and the second presentation occurs at the same time S forall of the substreams. In an alternative implementation, differentswitching times are employed for each substream. Referring again to FIG.14, it can be seen that the bandwidth requirements for the firstpresentation are reduced as the transmission of each segment iscompleted. These reduced bandwidth requirements can be used tosimultaneously ramp up the bandwidth that becomes available for thesecond presentation. To illustrate, FIG. 18 depicts the timerelationship of the substreams for an encoded transmission. In thisdiagram, time t=0 is the moment at which the subscriber presses the“play” button. As soon as this event occurs, copies of the fragments arecaptured from all of the various substreams. In particular, thecapturing operation does not need to wait until the first fragment ofthe substream appears on its designated substream. The fragments areretrieved in the order in which they appear, beginning at time 0.Furthermore, the playback does not begin as soon as the first fragmenthas been received. Rather, to guarantee uninterrupted display of thepresentation, the playback of a segment i start begins after one fullcopy of that segment has been transmitted over the time periodτ(1+λ)^(i) from time 0. Thus, the playback of the first segment beginswhen the time period τ has elapsed, at time t₀, once one copy of eachfragment in that segment has been received.

For any given segment i, a full copy of that segment is received betweentime 0 and time t_(i), and plays from time t_(i) to time t_(i+1), whichdefines an interval of duration λt_(i). However, each segment istransmitted over the interval of time τ(1+λ)^(i) which is as long as thesum of the viewing times of all preceding segments, plus the wait timeτ. Thus, each segment can be transmitted much more slowly than its ownviewing time.

In the layered implementation, the first required copy of each fragmentappears in the conventional layer. Consequently, the transmission of anencoded segment i need not begin until time t_(i+1) in order to meet theconditions of relationship (2). As a result, the substreams of thesecond presentation can start with an irregular boundary thatcomplements the irregular termination boundary of the firstpresentation, as depicted in FIG. 19, with sufficient room between themto avoid problems. Specifically, substream i of the first presentationmust play until time t_(i), and substream i of the second presentationneed not start until time t_(i+1). If the two presentations haveidentical encodings, e.g., they are both movies of the same length, thenthe inter-substream gap between the end of the first presentation andthe beginning of the second presentation is λt_(i). Otherwise, the gapis (t_(i+1))−t_(i), where t_(i+1) is determined relative to the encodingof the second presentation, and t_(i) is determined with respect to theencoding of the first presentation.

An exemplary encoding for a movie which incorporates the foregoingconcepts will now be described. As in the example of Table 1, a two-hourmovie is considered. The movie is divided into fragments of 188 byteseach, resulting in approximately 14.4 million fragments. The value for λis chosen to be 1/25, and the bandwidth multiplier is approximatelyequal to 8. Consequently, 25 substreams are allocated to theconventional transmission of the movie, and the number of encodedsubstreams N=175. Pursuant to Equation 8, the wait time t is a functionof the movie length, and is approximately 7.5 seconds. Given theseparameters, therefore, the first two segments of the movie contain 602and 627 fragments, respectively.

The encoding of the movie is created by time-division multiplexing the175 encoded substreams with the 1/λ=25 conventional substreams. As inthe example of Table 1, the individual substreams are represented by therows of the table, and each column represents one transmission slot. Forthe conventional transmission, the fragments of the presentation areallocated to the 25 substreams in raster order, as depicted in Table 2below.

TABLE 2 Substream Fragment Sequence 0 0 25 50 75 100 125 150 175 200 225• • • 1 1 26 51 76 101 126 151 176 201 226 • • • 2 2 27 52 77 102 127152 177 202 227 • • • 3 3 28 53 78 103 128 153 178 203 228 • • • 4 4 2954 79 104 129 154 179 204 229 • • • 5 5 30 55 80 105 130 155 180 205 230• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

The encoded substreams begin at Substream 25 in this example. Theassignment of the fragments to the encoded substreams is carried outwith reference to the fragments in the conventional substreams. Thefirst encoded substream contains fragments 0 through 601. To determineits appropriate location, fragment 601 is identified in the conventionalsubstream. Beginning in the next column, i.e. transmission slot, of theencoded substream, fragments 0-601 are sequentially inserted, and thissequence is repeated until one segment length beyond the end of theconventional data stream. This encoding is depicted in Table 3 below.

TABLE 3 Fragment Sequence Conventional 500 525 550 575 600 625 650 675700 725 • • • Substreams 501 526 551 576 601 626 651 676 701 726 • • •0-24 502 527 552 577 620 627 652 677 702 727 • • • • • • • • • • • • • •• • • • • • • • • • • • • • • • • • • • • • 523 548 573 598 623 648 673698 723 748 • • • 524 549 574 599 624 649 674 699 724 749 • • •Substream 25  0  1  2  3  4 • • • Conventional 15,600 15,625 #####15,675 ##### ##### • • • Substreams 15,601 15,626 ##### 15,676 ########## • • • 0-24 15,602 15,627 ##### 15,677 ##### ##### • • • • • • • •• • • • • • • • • • • • • • • • • • • • • • 15,623 15,648 ##### 15,698##### ##### 15,624 15,649 ##### 15,699 ##### ##### Substream 25   599  600 601    0 1 2 • • •

The foregoing examples are presented for the case in which the bandwidthrequirements are uniform, such that the temporal spacing of fragments isthe same for all segments of the presentation. As noted previously,however, the data rates, and hence the bandwidth requirements, can varyin dependence upon the content of the presentation. As a result, onesegment of the presentation may require a significantly greater numberof fragments to be transmitted in a given time than another segment ofthe movie. When the fragments of these two segments are multiplexed,consideration must be given to these differing data rates.

In accordance with another aspect of the present invention, a techniqueidentified as periodic-queue time-division multiplexing is employed toaccommodate variable rate data streams. Each substream comprises aseries of events, where each event is the beginning of the transmissionof a fragment. Each event is associated with a nominal time, and withina given substream the nominal times occur at regular intervals. Thelength of that interval, which is therefore equal to the temporal lengthof a fragment, can be expressed as follows:

$\begin{matrix}{{\Delta\; t_{i}} = \frac{t\left( n_{i} \right)}{n_{i + 1} - n_{i}}} & (28)\end{matrix}$

where: t(n_(i)) is the temporal length of segment i, and

-   -   n_(i+1)−n_(i) is the number of fragments in segment i.        The value of Δt_(i) can be different for different values of i,        i.e. different substreams, in the case of variable bandwidths.

The objective of periodic-queue time-division multiplexing is to assigneach event an actual transmission time such that, when the events fromall of the substreams are multiplexed together, the actual times occurat regular intervals. This is accomplished by assigning each fragment anominal broadcast time. FIG. 20 illustrates the nominal times that areassigned to the fragments in three segments having different nominaltime intervals Δt_(i). Once the nominal times have been assigned, thefragments are sorted in order of nominal time. In the case of ties, thefragments with the same nominal time can be arbitrarily ordered, e.g.,in accordance with their segment number. The fragments are thentransmitted in the sorted order at equally spaced actual times, toprovide a fixed data rate. The interval Δt_(max) between the actualtimes is chosen as the harmonic mean of the nominal time intervals:

$\begin{matrix}{\frac{1}{\Delta\; t_{mux}} = {\sum\limits_{i}\frac{i}{\Delta\; t_{i}}}} & (29)\end{matrix}$

As a result, each of the fragments is broadcast, on average, at theappropriate frequency. Furthermore, the difference in the nominal-timeinterval between any two events, e.g. successive copies of the samefragment, and the actual-time interval between those same events isbounded. For any two events whose nominal times are separated by aduration t, the number of intervening events that occur within eachsubstream k is an integer (t/Δt_(k))+δ_(k), where δ_(k) is a real-valuednumber in the range (−1, +1). The total number of intervening eventsfrom the entire data stream is therefore (t/Δt_(max))+Σ_(k)δ_(k).Consequently, the actual-time difference between two events differs fromthe nominal time difference, t, by the quantity t/Δt_(max)Σ_(k)δ_(k).This quantity lies within the range (−NΔt_(max), +NΔt_(max)). In otherwords, for any two events, the difference between the actual and nominaltimes is bounded by N units of Δt_(max). Hence, the cost associated withthe ability to provide a constant bandwidth data stream is an increaseof the actual wait time, beyond the nominal wait time τ, by a quantity(N−1)Δt_(max), where Δt_(max)=G/ηm. For the example described inconnection with Tables 2 and 3, this amounts to an increase of about12.5 ms.

When a program switch occurs, the number of substreams N might change ifthe two programs are not of the same length. Further, if the embodimentof FIG. 19 is employed, the various substreams stop and start atstaggered times. The periodic-queue time-division multiplexing techniqueshould be applied in a manner which takes these factors intoconsideration. To do so, a grid such as those of Tables 2 and 3 isdefined, having an infinite number of columns and a number of rows equalto the maximum anticipated value of N. The fragments of thepresentations are then scan-converted into the grid in order of nominaltimes, using only those cells which correspond to substreams that arecurrently active. Referring to FIG. 19, for example, during the periodfrom time 0 to t₀, no fragments are placed in the rows of the grid whichpertain to the conventional substreams, i.e. rows 0-24 in the example ofTable 3. At time t₀, the fragments of the second conventionalpresentation begin to be loaded. At this same time, encoded fragmentsfrom the first presentation are no longer loaded into the next row, e.g.row 25, although they continue to be loaded into all subsequent rows. Attime t₁, the fragments of the second presentation begin to be loadedinto row 25, and fragments of the first presentation are no longerloaded into row 26. The process continues in this manner, until acomplete switch from the first to the second presentation has takenplace.

From the foregoing, therefore, it can be seen that the present inventionprovides the ability to broadcast temporally ordered data in a mannerwhich enables a user to utilize that data with a minimal waiting period,regardless of the point in time at which the request to utilize the datais made. In its application to the transmission of video presentations,a user is able to begin viewing a two hour movie with a maximum waittime of approximately 7.5 seconds. Using currently availabletransmission techniques, these results can be accomplished within thebandwidth that is allocated to a conventional television channel, whileat the same time permitting a conventional transmission of the movie totake place as well. Hence, every available television channel is capableof providing a different movie in both a conventional mode and avideo-on-demand mode. The number of different movies that can be viewedin the video-on-demand mode is not limited by the storage capacities atthe viewer's site. Rather, it is determined only by the number ofchannels that the television service provider allocates to the movies.

The efficiency of the present invention can be readily ascertained byreference to two dimensionless parameters, bandwidth ratio and timeratio. The bandwidth ratio η is a factor by which the bandwidth of anencoded transmission exceeds that of a conventional broadcast of thesame presentation. This is a measure of the cost to the broadcaster. Thetime ratio T/τ is the factor by which the presentation length T exceedsthe wait time τ, and is therefore a measure of the benefit to theviewer. The graph of FIG. 21 illustrates the relationship of these tworatios for three cases, near-video-on-demand (curve 42), pyramidbroadcasting as described in the previously cited references (curve 44),and the present invention (curve 46). As can be seen, fornear-video-on-demand the time ratio is linear in the bandwidth ratio.Pyramid broadcasting provides improved results. For example, a bandwidthratio of 10 has a time ratio of 62, and a bandwidth ratio of 16 has atime ratio of 248. In the present invention, a bandwidth ratio in therange of 6-8 provides a time ratio of 400-3000, e.g. two-to-twelvesecond wait times for a two-hour presentation.

The features of the present invention can be combined with other knowntechniques to provide additional enhancements. For instance, thepreviously described approach, in which the beginning portions of moviesare stored ahead of time at the subscriber's premises for instantretrieval, can be employed in conjunction with the invention toeliminate the wait time τ altogether. In this case, however, only a verysmall portion of the movie needs to be preliminarily stored at thesubscriber's premises, e.g. 7.5 seconds for a two-hour movie, therebysignificantly reducing the local storage requirements per movie. As analternative, material which is common to several presentations, such asa copyright warning, a rating announcement, a studio logo, or anadvertisement, can be locally stored and played during the initial τseconds, to eliminate the perception of any waiting time.

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Forinstance, the foregoing examples of the invention have been describedfor the cases in which each segment contains the maximum integral numberof fragments that fit within the repetition period of the segment. Inthis case, in order to ensure that each fragment arrives at thesubscriber's premises within the required time, the fragments aretransmitted in the same sequence within each fragment. However, if it isdesirable to be able to reorder the fragments from one segment to thenext, it is possible to transmit less than the maximum number offragments within the period of a segment. In this case, the extra spacethat is available within the segment can be used to reorder thefragments, e.g. transmit a given fragment before it is required, whilestill preserving the time guarantees.

Similarly, while the use of equal-size fragments has been described inthe foregoing examples, particularly to facilitate the multiplexing ofthe substreams, other embodiments of the invention might employfragments of varying sizes, for instance where the substreams aretransmitted independently of one another, in parallel.

Furthermore, while the foregoing embodiments of the invention have beenspecifically described with reference to their application to televisedmovies, it will be appreciated that the principles which underlie theinvention can be applied to any type of temporally-ordered data that isdistributed via a broadcast or multicast medium. The presently disclosedembodiments are therefore considered in all respects to be illustrative,and not restrictive. The scope of the invention is indicated by theappended claims, rather than the foregoing description, and all changesthat come within the meaning and range of equivalence thereof areintended to be embraced therein.

We claim:
 1. A computer readable storage device storing computerexecutable instructions that when executed by a computer perform amethod for delivering streaming digital media, the method comprising:dividing, using a processor, the streaming digital media into a sequenceof segments; dividing, using the processor, the sequence of segmentsinto successive fragments, wherein the streaming digital media includesvideo or audio program data; assigning a nominal transmission time foreach fragment within a segment, wherein the nominal transmission time isbased on a time interval that is a function of a length of the segmentand the number of fragments in the segment; sorting the fragments inaccordance with the nominal transmission time assigned for eachfragment; and transmitting the successive fragments in the sorted orderat a fixed data rate, wherein the fixed data rate is based on a harmonicmean of the time intervals for the segments, wherein a time betweentransmission of each of the successive fragments is equal to theharmonic mean of the time intervals for the segments, and wherein thesuccessive fragments are delivered for replaying the video or audioprogram data.
 2. The method of claim 1 wherein a given segment of thesuccessive fragments is transmitted over a period of time equal to thepresentation time of all previous segments plus a predetermined waitperiod.
 3. The method of claim 1, further comprising deleting one ormore fragments in at least one segment.
 4. The method of claim 1 whereintransmitting the successive fragments includes transmitting thesuccessive fragments in a manner that ensures a maximum wait period τbefore one or more televisions or display devices can commencedisplaying the media.
 5. The method of claim 4 further comprisingdetermining an optimized bandwidth for a given wait period τ, whereindetermining an optimized bandwidth includes iteratively repeating aprocess including 1) calculating locally optimal values for a firstsubset of segments and 2) calculating locally optimal values for asecond subset of segments.
 6. A system for delivering streaming digitalmedia, the system comprising: means for dividing the streaming digitalmedia into a sequence of segments; means for dividing the sequence ofsegments into successive fragments, wherein the streaming digital mediaincludes video or audio program data; means for assigning a nominaltransmission time for each fragment within a segment, wherein thenominal transmission time is based on a time interval that is a functionof a length of the segment and the number of fragments in the segment;means for sorting the fragments in accordance with the nominaltransmission times assigned for each fragment; and means fortransmitting the successive fragments in the sorted order at a fixeddata rate, wherein the fixed data rate is based on a harmonic mean ofthe time intervals for the segments, wherein a time between transmissionof each of the successive fragments is equal to the harmonic mean of thetime intervals for the segments, and wherein the successive fragmentsare delivered for replaying the video or audio program data.
 7. Thesystem of claim 6 wherein a given segment of the successive fragments istransmitted over a period of time equal to the presentation time of allprevious segments plus a predetermined wait period.
 8. The system ofclaim 6, further comprising means for deleting one or more fragments inat least one segment.
 9. The system of claim 6 wherein transmitting thesuccessive fragments includes transmitting the successive fragments in amanner that ensures a maximum wait period τ before one or moretelevisions or display devices can commence displaying the media. 10.The system of claim 6 further comprising means for determining anoptimized bandwidth for a given wait period τ, wherein determining anoptimized bandwidth includes iteratively repeating a processincluding 1) calculating locally optimal values for a first subset ofsegments and 2) calculating locally optimal values for a second subsetof segments.
 11. A method for delivering streaming digital media, themethod comprising: dividing the streaming digital media into a sequenceof segments; dividing the sequence of segments into successivefragments, wherein the streaming digital media includes video or audioprogram data; assigning a nominal transmission time for each fragmentwithin a segment, wherein the nominal transmission time is based on atime interval that is a function of a length of the segment and thenumber of fragments in the segment; sorting the fragments in accordancewith the nominal transmission times assigned for each fragment; andtransmitting the successive fragments in the sorted order at a fixeddata rate, wherein the fixed data rate is based on a harmonic mean ofthe time intervals for the segments, wherein a time between transmissionof each of the successive fragments is equal to the harmonic mean of thetime intervals for the segments, and wherein the successive fragmentsare delivered for replaying the video or audio program data.
 12. Themethod of claim 11 wherein a given segment of the successive fragmentsis transmitted over a period of time equal to the presentation time ofall previous segments plus a predetermined wait period.
 13. The methodof claim 11, further comprising deleting one or more fragments in atleast one segment.
 14. The method of claim 11 wherein transmitting thesuccessive fragments includes transmitting the successive fragments in amanner that ensures a maximum wait period τ before one or moretelevisions, or display devices can commence displaying the media. 15.The method of claim 11 further comprising determining an optimizedbandwidth for a given wait period τ, wherein determining an optimizedbandwidth includes iteratively repeating a process including 1)calculating locally optimal values for a first subset of segments and 2)calculating locally optimal values for a second subset of segments.